Investments of Operating Entities and Consolidated Funds - Securities Owned at Fair Value - (Details) - USD ($)		9 Months Ended		12 Months Ended
		Sep. 30, 2016		Dec. 31, 2015
Investment Holdings [Line Items]
Fair Value, Concentration of Risk, Investments		$ 0		$ 0
Marketable Securities, Consolidated Funds		71,456,000		32,000,000
Securities owned, at fair value		544,644,000		610,234,000
US Government Securities
Investment Holdings [Line Items]
Marketable Securities, Consolidated Funds		3,999,000	[1]	0
Trading Securities, Debt	[2]	$ 3,776,000		$ 3,016,000
Debt securities, interest rate		0.00%
US Government Securities | Minimum
Investment Holdings [Line Items]
Debt securities, interest rate				0.00%
US Government Securities | Maximum
Investment Holdings [Line Items]
Debt securities, interest rate				5.95%
Preferred Stock
Investment Holdings [Line Items]
Marketable Securities, Consolidated Funds		$ 37,243,000		$ 32,000,000
Trading Securities, Equity	[3]	12,964,000		25,563,000
Common Stock
Investment Holdings [Line Items]
Marketable Securities, Consolidated Funds		22,950,000		0
Trading Securities, Equity	[3]	504,093,000		516,108,000
Convertible Bonds
Investment Holdings [Line Items]
Trading Securities, Debt	[4]	$ 250,000		$ 819,000
Debt securities, interest rate		8.00%
Convertible Bonds | Minimum
Investment Holdings [Line Items]
Debt securities, interest rate				8.00%
Convertible Bonds | Maximum
Investment Holdings [Line Items]
Debt securities, interest rate				10.00%
Corporate Bonds
Investment Holdings [Line Items]
Marketable Securities, Consolidated Funds		$ 5,452,000	[5]	$ 0
Trading Securities, Debt	[6]	$ 17,794,000		$ 47,192,000
Corporate Bonds | Minimum
Investment Holdings [Line Items]
Debt securities, interest rate		3.25%		3.25%
Corporate Bonds | Maximum
Investment Holdings [Line Items]
Debt securities, interest rate		13.00%		9.00%
Warrants and Rights
Investment Holdings [Line Items]
Marketable Securities, Consolidated Funds		$ 3,000		$ 0
Trading Securities, Equity		5,762,000		3,059,000
Mutual Funds
Investment Holdings [Line Items]
Trading Securities, Equity	[7]	5,000		14,477,000
Loans
Investment Holdings [Line Items]
Marketable Securities, Consolidated Funds		$ 1,809,000		0
Consolidated Funds | Corporate Bonds | Minimum
Investment Holdings [Line Items]
Debt securities, interest rate		5.88%
Consolidated Funds | Corporate Bonds | Maximum
Investment Holdings [Line Items]
Debt securities, interest rate		14.37%
Consolidated Funds | US Government Agencies Debt Securities [Member]
Investment Holdings [Line Items]
Debt securities, interest rate		0.00%
Enterprise Master
Investment Holdings [Line Items]
Securities owned, at fair value		$ 2,546,000		2,781,000
Enterprise Master | Restricted Stock
Investment Holdings [Line Items]
Securities owned, at fair value		124,000		124,000
Enterprise Master | Preferred Stock
Investment Holdings [Line Items]
Securities owned, at fair value		1,587,000		1,484,000
Enterprise Master | Common Stock
Investment Holdings [Line Items]
Securities owned, at fair value		835,000		724,000
Enterprise Master | Rights
Investment Holdings [Line Items]
Securities owned, at fair value		0		321,000
Enterprise Master | Trade Claims
Investment Holdings [Line Items]
Securities owned, at fair value		0		128,000
Merger Master
Investment Holdings [Line Items]
Securities owned, at fair value		665,142,000		157,921,000
Merger Master | Common Stock
Investment Holdings [Line Items]
Securities owned, at fair value		650,699,000		157,429,000
Merger Master | Corporate Bonds
Investment Holdings [Line Items]
Securities owned, at fair value		$ 14,443,000		$ 492,000	[8]
Debt securities, interest rate		6.50%		5.25%
Caerus Master [Member] | Common Stock
Investment Holdings [Line Items]
Securities owned, at fair value		$ 5,300,000
Common Stock
Investment Holdings [Line Items]
Trading Securities, Equity	[3]			$ 7,400,000
Preferred Stock
Investment Holdings [Line Items]
Trading Securities, Equity	[3]	$ 4,200,000		7,700,000
Affiliated Entity | Mutual Funds
Investment Holdings [Line Items]
Trading Securities, Equity	[7]			$ 13,400,000

[1] As of September 30, 2016, maturity was December 2016 with an interest rate of 0%.

[2] As of September 30, 2016, maturities ranged from December 2016 to August 2017 with an interest rate of 0%. As of December 31, 2015, maturities ranged from January 2016 to August 2016 with interest rates ranged between 0% to 5.95%.

[3] Included in preferred stocks are investments in securities for which the Company has elected the fair value option with the fair value of $4.2 million at September 30, 2016 and preferred and common stock of $7.7 million and $7.4 million, respectively, at December 31, 2015. These investments were acquired in connection with merchant banking transactions.

[4] As of September 30, 2016, the maturity was March 2018 with an interest rate of 8%. As of December 31, 2015, maturities ranged from July 2016 to March 2018 with interest rates ranged between 8% to 10.00%.

[5] As of September 30, 2016, maturities ranged from October 2017 to May 2049 and interest rates ranged between 5.88% and 14.37%.

[6] As of September 30, 2016, maturities ranged from October 2016 to February 2046 and interest rates ranged between 3.25% to 13.00%. As of December 31, 2015, maturities ranged from March 2016 to February 2046 and interest rates ranged between 3.25% to 9.00%.

[7] Included in this amount as of December 31, 2015, are investments in affiliated funds of $13.4 million all of which was liquidated during the three months ended March 31, 2016.

[8] As of September 30, 2016, the maturity was January 2024 with an interest rate of 6.5%. As of December 31, 2015, the maturity was June 2024 with an interest rate of 5.25%.